Y'know, I was actually thinking about this question for a long time, because I thought I had an answer right off the bat, and when I realized I didnt...
Well, I'm pretty happy with my answer now. I might change it in my mind later, but I think it's more honest than making any clear-cut distinction of exactly who or what you'd save.
Actually, one of my first responses was just 'it depends upon the situation'. Another was 'Well, since a life is the most precious thing, I'd give my one up for two others to survive'. But both of these didn't feel quite right. My other attempt was 'anyone I love', but that's far too vague. So...
Whatever can kill me.

Hmm i seem to sacrifice myself quite often not to the extent of possible death.One time my friend and i got hit by a car 2 years ago while walking to the store a few blocks from her house. My hip took quite a bit damage but non the less i still carried her to her house and from there her mom drove her to the hospital as i walked home.(I dislike hospitals so i didn't go)